Fiber optic cables (i.e., optical cables) are commonly used for data transfer and communications in a variety of networking applications. The typical “loose tube” fiber optic cable contains at least one buffer tube, which in turn contains at least one optical fiber. Single tube cables are called “central” loose tube cables, while cables with multiple buffer tubes are called “stranded” loose tube cables.
With any type of fiber optic cable system data transmission capabilities can deteriorate due to water migration. In particular, water migration damages the integrity of the system components over time, leading to weaker transmission signals or total fiber breakage. Due to the applications for fiber optic cables in environments that may include water, the fiber optics industry emphasizes reducing water migration problems to ensure proper functionality and longer lifespan of fiber optic cable systems.
The compounds used for blocking water migration within a fiber optic cable generally separate into two classifications: (1) “filling” compounds and (2) “flooding” compounds. Filling compounds, discussed at length herein, are placed within the buffer tube. Conversely, flooding compounds block water migration outside the buffer tubes, such as between the buffer tubes and the overall jacketing layers, including the interstices.
As reported in U.S. Pat. No. 7,006,740, filling compounds used within the buffer tubes are typically distinct materials from flooding compounds used to fill the interstices outside the buffer tube. For instance, flooding compounds are often not appropriate to use within the buffer tube because the material has other properties not suitable or desirable for the filling application. Therefore, those skilled in the art distinguish between flooding compounds and filling compounds when selecting materials for the interior of a buffer tube.
The techniques described in the art for blocking water migration within buffer tubes substantially separate into three general categories: (1) using thixotropic gels as a filling compound, (2) adding super absorbent powder (SAP) within the buffer tube, and (3) adding material to create water-blocking zones within the buffer tube. All three of these solutions have their drawbacks, described below.
Thixotropic gel, as described in U.S. Pat. No. 6,847,768, is a conventional filling compound and can prevent water migration inside buffer tubes. The buffer tubes are filled with the gel, which physically blocks any flow of water longitudinally along the cable. The gel also minimizes micro bending effects by acting as a cushion for the fibers or fiber ribbons. Additionally, thixotropic gels and can help couple the fibers to the buffer tube, which is helpful in limiting movement relative to the buffer tube and cable, preventing attenuation of the optical signals within the fibers.
However, using the gels has its drawbacks. The gel should be removed from the fibers before connecting or splicing the fibers, as disclosed, for example, in U.S. Pat. No. 7,277,615. This operation is not only time consuming, but also messy. Contamination of the work space is possible and the use of consumables such as wipes and gel cleaning compounds can present disposal problems.
More recently, the optical fiber cable industry has moved to “dry” cables, which use buffer tubes including tapes or yarns that contain super absorbent powders (SAP) rather than thixotropic gels. Water-swellable tapes and yarns have also been used to block water outside the buffer tubes underneath the overall jacket. Cables using SAP elements both inside the buffer tubes and outside the buffer tubes are known as “dry-dry” cables. These cables are described, for example, in U.S. Pat. No. 4,909,592, incorporated herein by reference. That patent describes using water-swellable yarns, one type of SAP element, both inside and outside the buffer tubes to block water migration. Another example of a dry-dry optical fiber cable is disclosed in U.S. Pat. No. 6,970,629. This patent teaches the use of a compressed dry insert, made of a foam tape and a water-swellable tape, which surrounds the optical fiber ribbon in a single tube cable. The normalized ribbon pullout force taught by U.S. Pat. No. 6,970,629 is between 0.5 N/m and 5.0 N/m and more preferably from about 1 N/m to about 4.0 N/m.
Although SAP elements block water migration without the cleanup drawbacks associated with gels, SAP elements present disadvantages. For example, some SAP elements provide little coupling of the optical fibers to the inside of the buffer tube, and do not provide the cushioning effect that gels do. Without cushioning, the optical fibers are more susceptible to damage and optical loss. Additionally, poor coupling inherent in such dry-dry cables can give rise to attenuation increases because the optical fibers can be irreversibly drawn into the buffer tube if the cable stretches during installation. This could increase the fiber to buffer tube length ratio, which gives rise to attenuation at low temperatures.
SAP elements also take up space in the buffer tube, which can negatively restrict movement, present microbending attenuation effects, and require an increase in the diameter of the tubes. Additionally, because the coupling effects of SAP elements may not be as great as the gels, those skilled in the art often try to increase the coupling characteristics by using many more SAP elements than necessary for water-blocking, as reported in U.S. Pat. No. 7,277,615. But this is likely to exacerbate the other disadvantages of SAP elements described above.
Another concern with the use of SAP elements is that the swelling efficiency of the powder used in the water-blocking elements decreases with increasing ion concentration in the water, as reported in 1997 NFOEC paper, “Performance of Swellable Materials in High Ionic and Seawater environments,” by J. Davis and R. Demaree. Thus, these “dry” cables can be less effective at blocking water, and require more (or larger) SAP elements, as the ionic concentration of the water increases. Considering that optical fiber cables are routinely exposed to the high ion content of dilute seawater, runoff containing road salt, and other sites, this characteristic can detract from reliable performance.
The third type of solution for preventing water migration involves providing water-blocking plugs at spaced intervals throughout the buffer tube, as described for example in U.S. Pat. Nos. 6,463,199 and 6,847,768, both of which are incorporated by reference. The water-blocking plugs are made of solid materials, physically blocking water migration.
The use of water-blocking plugs as disclosed in the art has several disadvantages. First, the introduction of the solid water-blocking plugs during manufacturing is not trivial, and can be quite expensive. Current water-blocking plugs are solid, and are difficult to insert inside the buffer tube such that they surround the optical fiber within the buffer tube. To this end, U.S. Pat. No. 6,847,768 teaches (1) cutting plugs from a continuous length of material and attaching the plugs to a portion of the buffer tube assembly during manufacture or (2) injecting the solid plug material. To prevent the collapse of the buffer tubes during manufacture, U.S. Pat. No. 6,931,184 reports on the practice of injecting thixotropic gel inside the buffer tube during the extrusion of the buffer tube around the optical waveguide, or, alternatively, using a special buffer material. In the presence of a buffer tube made of standard buffer materials that is neither fully filled, i.e. with a grease or gel, nor partially filled with continuously injected SAP elements, the buffer tube can collapse during extrusion if the plug material is not continuous and is injected during manufacture.
Second, like the aforementioned SAP elements, some of the solid water-blocking plugs do not provide for strong coupling between the optical fibers and the buffer tube. This is by design, because the hard, relatively inflexible, nature of current water-blocking plugs would cause attenuation or microbending effects if coupled too strongly to the fibers and buffer tube. As a result, the strongest coupling in cables described in the art is a normalized pullout force of less than 5.0 N/m.
“Normalized pullout force” is the pullout force measured in a certain cable length referred to as a unitary cable length.
Even though a pullout force higher than 5.0 N/m provides certain advantages, it has negative consequences on attenuation and microbending under current techniques.
Pullout force is the force required to begin movement of the optical fibers and/or ribbons longitudinally from the end of the cable, and serves as an indication of how well the fibers are coupled to the buffer tube and the cable. To meet typical customer specifications, the pullout force must be greater than 0.1625 N times the number of fibers in the ribbon stack over a 30 meter test length. Thus, for a 144 fiber ribbon stack (12 stacks of 12-fiber ribbons), the pullout force must be greater than 23.4 N, which normalizes to 0.78 N/m. For a 72-fiber ribbon stack, the pullout force must be greater than 11.7 N per 30 meters (which normalizes to 0.39 N/m.) Cables described in the art, when equipped with any of the hereinbefore described water-blocking plugs, typically exhibit normalized pullout forces of less than 5 N/m.
The pullout force is important for long-term network reliability due to elevation changes and the tendency for displacement, particularly at the low points of the cable. Adequate pullout force is also important to minimize or eliminate fiber movement during installation. Bend-induced attenuation, pulling fibers from splice trays, or, in the worst case, fiber breaks can occur due to poor fiber coupling. For central tube ribbon cables, coupling the fibers to the buffer tube is especially important because single-tube designs do not use stranded tubes (which inherently lock ribbons or fibers in place).
Third, the water-blocking plug as disclosed in U.S. Pat. No. 6,847,768, for example, provides a passage for an optical fiber ribbon stack that is slightly larger than the ribbon stack, thus providing a path of water migration. Therefore a water-swellable tape, powder, or layer may be required in the passageway around the optical fiber ribbon stack. Further, as described in U.S. Pat. No. 6,463,199, in order to cleanly strip the water-blocking plug from the optical fiber, some of the water blocking plugs described in the art may need a release layer or a controlled bond layer between the optical fiber and the water-blocking plug.
An example of one or more dry inserts blocking water and coupling the optical fibers to the tube is provided by U.S. Pat. No. 7,277,615. This patent explains that the compression of the dry insert may be in the range from about 10% to about 90% and that the ribbon pullout force is in the range of about 0.5 N/m and about 5.0 N/m, more preferably, in the range of about 1 N/m to about 4 N/m. This document teaches that the normalized pullout force for an optical ribbon in a conventional cable employing thixotropic grease or gel is about 4.8 N/m, and that ribbons in dry cables using only non-compressed SAP elements have pullout forces of 0.5 N/m or less. The normalized pullout force for a ribbon in the cables of U.S. Pat. No. 7,277,615 is between 0.5 N/m and 5.0 N/m. This art reflects the current difficulties in attaining a pullout force greater than 5.0 N/m through use of conventional water-blocking plugs, without raising the risk of microbending and attenuation to an unacceptable level.
It is also worth noting that these pullout forces can be measured with respect to the entire cable. For a central tube cable embodiment, the pullout force with respect to the cable can be about the same as the pullout force with respect to a buffer tube. For a stranded cable embodiment, the pullout force with respect to the cable can be greater than the pullout force with respect to one of the buffer tubes.
Applicants have identified a need for an optical cable (and method of making the same) comprising a buffer tube and water-blocking plugs (i.e., elements) that can effectively block water migration, including water with high ionic concentration, within the buffer tube and maintain a strong coupling bond, while minimizing the above shortcomings. In particular, Applicants have identified a need for a buffer tube with internal water-blocking provisions that may: (1) provide for effective coupling between the optical fibers to the buffer tube, (2) cushion the optical fiber(s) inside the buffer tube, (3) provide thermal stability for the transmission integrity of the optical data, (4) reduce the mess involved when accessing the optical fibers in the field, and (5) be manufactured more cheaply and easily.
More particularly, the Applicant identified the problem of having a high pullout force of an optical fiber from a buffer tube without subjecting the optical fiber to stress due to cable movement (e.g., during laying and/or operation of the cable).
The Applicant found that the above-mentioned problems could be solved by water-blocking plugs accomplishing an adhesion to optical fiber and buffer tube material such to provide a high pullout force, said water-blocking plugs having a deformability such to accommodate the optical fiber with respect to the movements (bend, pull, thermal deformation) of the cable without stress thereto.